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The cornea’s main task is to protect the delicate<br />
components of the eye against damage by foreign<br />
bodies. The iris is located between the cornea and<br />
the lens, and its function is to control the amount<br />
of incident light, in the same way as the diaphragm<br />
of a camera. The lens focuses the incoming<br />
light rays on to the retina (Latin: rete = net),<br />
where the actual process of perception begins.<br />
The photo-receptors (the rods and cones) convert<br />
the incident ’optical signals’ into chemical (and<br />
subsequently into electrical) signals. These electrical<br />
signals then travel to the brain along the<br />
optic nerve. There are no photo-receptor cells at<br />
the point where the optic nerve leaves the retina,<br />
and this is known as the “blind spot”. Another<br />
important feature of the retina is the so-called<br />
“yellow spot” (the macula lutea). In the middle of<br />
this is the fovea centralis, the point where visual<br />
acuity is at a maximum. There are no rods in this<br />
position, only cones, which are connected in a<br />
special way to the relevant nerve cells. When you<br />
focus your attention on a certain object, your<br />
head and eyes automatically move in such a way<br />
as to let its image fall on the fovea for greatest<br />
sharpness.<br />
The retina: The back of the eye can be observed<br />
through the pupil using an ophthalmoscope. The<br />
retina, with the blood vessels supplying its inner<br />
layers, can be seen, as well as the blind spot and<br />
the yellow spot.<br />
The retina plays a key role in visual perception.<br />
This thin (only 0.2 mm) layer of nerve tissue lines<br />
Canal of Schlemm<br />
Reflection of the covering<br />
layer (conjunctiva) from<br />
the front of the eye to the<br />
inside of the eyelid<br />
The ora serrata:<br />
Posterior to this<br />
point, the retina<br />
is light-sensitive<br />
Pupillary opening<br />
Cornea<br />
Anterior chamber<br />
Iris<br />
Posterior chamber with<br />
lens filaments<br />
Ciliary muscle<br />
Lens<br />
Choroid<br />
Retina<br />
The outer or ‘hard‘ layer<br />
of the brain’s covering<br />
membranes (Dura mater)<br />
Optic nerve<br />
Blind spot (optic papilla)<br />
External<br />
eye (lateral<br />
rectus) muscle<br />
Sclera<br />
Vitreous body<br />
Yellow spot (macula lutea,<br />
containing the central fovea)<br />
Visual axis<br />
14<br />
Transverse view of the right human eye<br />
(after Faller/Schünke, Der Körper des Menschen,<br />
Thieme-Verlag)
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